What is law of equivalence and where it is applied in solving the numericals

What is law of equivalence and where it is applied in solving the numericals.

According to law of chemical equivalence, for a chemical reaction:
aA + bB cC + dD
Equivalent of A = Equivalent of B = Equivalent of C = Equivalent of D
where n is n factor and M is molecular mass of ith species,
If the reaction is carried out in solution, then NAVA = NBVB = NCVC = NDVD